The present invention relates to energetic materials, particularly to the manufacture of energetic materials, and more particularly to the manufacture of energetic materials using sol-gel chemistry.
Energetic materials are herein defined as any material which stores chemical energy in a fixed volume. Explosives, propellants, and pyrotechnics are examples of energetic materials. Reaction results from either shock or heat. Explosives and propellants may be thought of as a means of storing gas as a “solid”. Pyrotechnics typically release much of their energy as heat.
Energetic materials consist of fuels and oxidizers which are intimately mixed. This is done by incorporating fuels and oxidizers within one molecule or through chemical and physical mixtures of separate fuel and oxidizer ingredients. The material may also contain other constituents such as binders, plasticizers, stabilizers, pigments, etc.
Traditional manufacturing of energetic materials involves processing granular solids into parts. These materials may be pressed or cast to shape. Performance properties are strongly dependent on particle size distribution, surface area of the constituents, and void volume. In many cases achieving fast energy release rates, as well as insensitivity to unintended initiation, necessitates the use of small particles (≦100 μm) which are intimately mixed. Reproducibility in performance is adversely affected by the difficulties of synthesizing and processing materials with the same particle morphology. Manufacturing these granular substances into complex shapes is often difficult due to limitations in processing highly solid filled materials.
An example of an existing limitation of processing granular solids is in manufacturing energetic materials for detonators. The state-of-the-art now requires the precise synthesis and recrystallization of explosive powders. These powders typically have high surface areas (e.g., >1 m2/g). The powders are weighed and compacted at high pressures to make pellets. Handling fine grain powders is very difficult. Dimensional and mechanical tolerances may be very poor as the pellets may contain no binder. Changes in the density and dimensions of the pellets affect both initiation and detonation properties. Manufacturing rates are also low as the process is usually done one at a time. Certification of material is typically done by expensive, end-use detonation performance testing and not solely by chemical and physical characterization of the explosive powder. As these detonators or initiating explosives are sensitive, machining to shape pressed pellets is typically not done.
Another current limitation is producing precise intimate mixtures of fuels and oxidizers. The energy release rates of energetic materials are determined by the overall chemical reaction rate. Monomolecular energetic materials have the highest power as the energy release rates are primarily determined by intramolecular reactions. However, energy densities can be significantly higher in composite energetic materials. Reaction rates (power) in these systems are typically controlled by mass transport diffusion rates.
The present invention solves many of these prior problems by manufacturing energetic materials through the use of sol-gel chemistry. Sol-gel chemistry is known in the art and broadly described here, but described hereinafter in greater detail to provide an understanding of this technology. In one approach, using sol-gel chemistry, a solution including explosive materials is made. That solution is then gelled to form a cross-linked skeleton, which may be either inert, or reactive, or energetic, and a continuous liquid phase. The liquid phase is then extracted to produce either a xerogel or an aerogel. A solid so produced has high surface area and homogeneity. During the solution stage of the operation, the solution may be easily cast into molds to produce final parts. By applying a stress during removal of the liquid phase, dense parts may be obtained. Alternately, explosive molding powders may be made which can be used as feedstock for pressing operations. The molding beads will have a high degree of uniformity in the microstructure. As these beads are orders of magnitude larger than explosive powders used in traditional processing they may be easily handled. Also, through using sol-gel chemistry the intimacy of mixing can be dramatically improved over mixing granular solids. Numerous synthetic routes may be carried out utilizing sol-gel chemistry in the processing of energetic materials; these include solution addition, powder/particle addition, nano-composites, and functionalized solid networks.